For some time the Applicant has marketed devices, in quantities distributed on a larger scale, where the electrolyte chamber has several volume-cubic centimeters of which only a few, e.g. 1 to 3 cubic centimeters, are used for filling. This small volume electrolyte chamber has an opening on the bottom with which the chamber is placed on a sample. The cathode is provided coaxially in the form of a hollow rod and, in order to maintain an approximately uniform electrolyte concentration, the device is equipped with a motor generating pulsed air pressure variations which are transmitted via the cathode to the volume so that the electrolyte rises and drops in synchronism with this impulse. For large volume electrolyte chambers such movement generation is not necessary since the volume is sufficiently large and the electrolyte is capable of circulating. Effective electrolyte flow does not exist in small volume electrolyte chambers, which always have an opening of the order of magnitude of several square millimeters.
For measurements of this type according to FIG. 1 of our above specified publication, the voltage increase must be as steep as possible also in those cases where an alloy layer is present between the layer and the substrate material, which is formed during thermal treatment. In one case Sn is the layer and iron is the substrate material. The alloy layer located between them is FeSn.sub.2.
The steepest slope is obtained when the alloy layer is completely uniformly removed, so that its thickness decreases uniformly everywhere, primarily in the final measuring phase.